Swedish scientist Ake Sellstrom told news agency TT that while he had only seen TV footage, the high number of casualties reported sounded suspicious.

"It sounds like something that should be looked into," he told TT by phone from Damascus. "It will depend on whether any UN member state goes to the secretary general and says we should look at this event. We are in place."

Britain also urged the Syrian government to allow the UN chemical weapons team, which is currently in Damascus, to be allowed access to the site, which is in the Ghouta region just outside of the capital.

Hague said reports of the attack, which has been denied outright by the Syrian government, remained uncorroborated and that Britain was urgently seeking more information.

"But it is clear that if they are verified, it would mark a shocking escalation in the use of chemical weapons in Syria," he said, adding that Britain would try to hold to account anyone who used chemical weapons or ordered their use.

"I call on the Syrian Government[6] to allow immediate access to the area for the U.N. team currently investigating previous allegations of chemical weapons use. The UK will be raising this incident at the U.N. Security Council," said Hague.

Britain provides non-lethal assistance to rebels fighting to overthrow Assad, but has stopped short of sending them arms and has seen its diplomatic efforts to put pressure on Assad frustrated by Russia and China at the United Nations.

"It is time for the UN Security Council to assume responsibility... by convening immediately to reach a clear deterrent decision that ends the humanitarian tragedy," said Foreign Minister Prince Saud al-Faisal.

"We ask EU foreign ministers meeting today in Brussels (to discuss the Egyptian crisis) to have this humanitarian catastrophe as the main topic of their talks," he told AFP.

French President Francois Hollande also called on the UN inspectors to visit the site of the attack, government spokeswoman Najat Vallaud-Belkacem said Wednesday.

During a cabinet session, Hollande "announced his intention to ask the UN to visit the site of the attack," she told reporters, adding: "This information obviously requires verification and confirmation."

Turkey too called on U.N. inspectors to look into the reports, and said it was monitoring the situation "with great concern".

"Light must immediately be shed on these claims and the United Nations mission that was formed to investigate chemical weapons claims in Syria should look into these claims and reveal its findings," the Foreign Ministry said in a statement.

"If these allegations are found to be true, it will be inevitable for the international community to take the necessary stance and give the necessary response to this savagery and crime against humanity," it said.

Ankara, once an Assad ally but now one of his fiercest critics, has long been concerned about the possible use of Syrian chemical weapons against its own people.

Turkey began stepping up tests on casualties arriving from Syria for treatment earlier this year to determine whether they were victims of a chemical weapons attack, but no details have been released publicly on the results of those tests.

The opposition Syrian National Coalition called for an urgent Security Council meeting.

"I call on the Security Council to convene urgently," National Coalition leader Ahmed al-Jarba told Al-Arabiya news channel, condemning the Syrian army's bombardment of the Ghouta suburbs of Damascus as a "massacre."